1. Field of the Invention
The present invention relates to multi-mode communications transmitters.
2. State of the Art
Different mobile communications systems are prevalent in different geographical regions. Example systems include those specified by the GSM and ANSI-136 standards, which are time division multiple access (TDMA) communication systems, the CDMA standard (IS-95), and combinations of the same (so-called multi-mode systems). Furthermore, the proliferation of competing standards is increasing with the adoption of different 2.5 and 3G mobile communications standards, such as EDGE, UMTS (WCDMA), CDMA2000, etc. Hence, although the vision of a “world phone” has been repeatedly articulated, progress toward that goal has been slow and difficult. Various current products offer multi-band operation for a particular standard. Fewer products offer multi-standard (i.e., multi-mode) operation. True multi-mode operation should enable mode switching to be done on-the-fly, in real time.
In TDMA communication systems, high quality RF (radio frequency) signals must ramp quickly from a condition of minimal output power to a condition of information-bearing modulation at a specified output power and back down to the condition of minimal output power. Such power ramping capability is illustrated in FIG. 1.
A fundamental requirement of these transmitters is that the acts of ramping up and ramping down must not violate specified limits on peak power in spectral bands away from the assigned RF channel (e.g., bands that would be allocated to other transmitters); the associated measurement is called the transient spectrum in some systems or the transient adjacent channel power (transient ACP) in others.
Present power ramping techniques must be tailored for each modulation type, and typically require unit-by-unit calibration (at least in the case of typical GMSK transmitters and conventional multi-mode transmitters). Even so, transient ACP performance is usually very sub-optimal.
Considerable challenges to true multi-mode operation where mode switching is done on-the-fly, in real time, are posed by the following problems: (a) how to fully ramp a communications signal down and then back up inside a guard period while switching from one mode to another so that signal glitches occur only while the signal is ramped down and so can be made negligible; or, alternatively, (b) how to change smoothly from one modulation to another without being fully ramped down.
Furthermore, if different hardware paths are used for the different modulations, then mode switching is even more difficult, as there may be switching transients when switching between the hardware paths, and these transients may or may not be controllable.
Other issues relate to signal quality, both in-band (as measured by error vector magnitude, or EVM) and out-of-band (as measured by power spectral density, or PSD). Maintaining high signal quality over a wide range of output powers for multiple standards poses a particular challenge.
The present invention is applicable to both conventional (I/Q) and polar modulation architectures. Polar modulation architectures, and similar architectures in which separate amplitude and phase paths are provided, are described, for example, in U.S. Pat. Nos. 6,191,653, 6,194,963, 6,078,628, 5,705,959, 6,101,224, 5,847,602, 6,043,707, and 3,900,823, as well as French patent publication FR 2768574, all of which are incorporated herein by reference.